1. Field of the Invention
The present invention relates to a novel ferritic stainless steel. It particularly includes a welded ferritic stainless steel and welded product having superior secondary working embrittleness resistance and superior high temperature fatigue characteristics, and concerns welded parts that are suitable for applications in which a welded pipe or a welded plate, after having undergone forming work, is used.
The expression xe2x80x9csecondary workingxe2x80x9d as used herein refers to the processing of a specified part after having already having subjected it to forming work. For example, a welded pipe may be subjected to bending work (primary working), and thereafter, to pipe diameter enlargement work (secondary working).
In known ferritic stainless steels, cracks due to brittleness are likely to form during secondary working.
The expression xe2x80x9chigh temperature fatiguexe2x80x9d as used herein refers to a phenomenon wherein fatigue fracture of a material occurs due to repetitive bending at high temperatures of 600xc2x0 C. or more.
For example, welded parts of components of an exhaust pipe system in an automobile undergo secondary working and high temperature fatigue. Among them, an exhaust manifold, as shown in FIG. 1 of the drawings, is subjected to severe conditions during operation, and undergoes intense vibration at high temperatures of 600xc2x0 C. or more due to the action of engine exhaust gas. This is a typical example. The present invention is preferably applied to, for example, an exhaust manifold of ferritic stainless steel, and other welded products.
2. Description of the Related Art
When a welded pipe that has been subjected to complicated bending work, or pipe diameter enlargement or reduction is used, for example, as an exhaust manifold of an automobile, problems arise because cracks occur in welded parts that had already become brittle due to secondary working. Fatigue cracks occur in welded parts during use, due to insufficient strength at a high temperature.
The primary reason cracks are likely to occur in welded parts, rather than base materials, is that the toughness and strength of the welded parts deteriorate because crystal grains of the welded parts become coarse due to heat input during welding.
A ferritic steel containing an intervening material, Al2O3, has been suggested in Japanese Unexamined Patent Publication No. 11-172369. However, the aforementioned kind of steel exhibits insufficient secondary working embrittleness which causes cracks in the welded parts. Whether or not high temperature fatigue characteristics are achieved, serious cracks frequently occur as a result of the harmful secondary working embrittleness.
In order to reduce an intervening material introduced into the steel, Al2O3, Si or Mn must be used as a deoxidizer in the steel making process. Accordingly, Al, widely used as a deoxidizer, cannot be used in production of welded products free of defects caused by harmful secondary working embrittleness.
A ferritic stainless steel having improved secondary working embrittleness resistance by adding phosphide, and controlling its size and amount, was suggested in Japanese Unexamined Patent Publication No. 7-126812. When P is added, however, degradation of toughness of the welded product cannot be avoided. It is believed that this is a result of segregation of P at the grain boundaries of the welded part, due to heat input during welding.
Furthermore, high temperature fatigue characteristics of a welded part are not improved by controlling the amount of phosphide. Accordingly, high temperature fatigue cracks cannot be prevented by the addition of P to the steel.
As described above, regarding improvements of secondary working embrittleness resistance and high temperature fatigue characteristics, various suggestions have been made. However, no ferritic stainless steel having both of these advantageous properties has been discovered.
It is an object of this invention to do so.
It is an object of the present invention to meet the aforementioned demand and to provide the significant advantages heretofore detailed.
It is a further object of the present invention to provide a ferritic stainless steel in which both secondary working embrittleness resistance and high temperature fatigue characteristic of welded parts are improved.
A ferritic stainless steel and a ferritic stainless steel welded part are provided with both superior secondary working embrittleness resistance and high temperature fatigue characteristic in accordance with this invention.
The ferritic stainless steel of this invention has a composition, on a weight percentage basis, composed of about: 0.02% or less of C., 0.2% to 1.0% of Si, 0.1% to 1.5% of Mn, 0.04% or less of P, 0.01% or less of S, 11.0% to 20.0% of Cr, 0.1% to 1.0% of Ni, 1.0% to 2.0% of Mo, 1.0% or less of Al, 0.2% to 0.8% of Nb, 0.02% or less of N, 0.01% to 0.3% of Co, 0.01% to 0.3% of V, 0.0002% to 0.0050% of B, and the remainder Fe and incidental impurities.
The ferritic stainless steel contents of Co, V, and B preferably fall within the range represented by the following formula
0.1xe2x89xa6[Co]+0.5xc3x97[V]+100xc3x97[B]xe2x89xa60.5
where [Co], [V] and [B] designate the contents by weight percentages of the respective elements.
The aforementioned ferritic stainless steel preferably has a composition, on a weight percentage basis, further comprising at least one element selected from the group consisting of about 0.05% to 0.5% of Ti, about 0.05% to 0.5% of Zr, and about 0.05% to 0.5% of Ta.
The aforementioned ferritic stainless steel preferably has a composition, on a weight percentage basis, further comprising about 0.1% to 2.0% of Cu.
The aforementioned ferritic stainless steel preferably has a composition, on a weight percentage basis, comprising at least one element selected from the group consisting of about 0.05% to 1.0% of W and about 0.001% to 0.1% of Mg.
The aforementioned ferritic stainless steel preferably has a composition, on a weight percentage basis, further comprising about 0.0005% to 0.005% of Ca.